Nondestructive stress measurements through prism coupling have great importance for quality control in manufacturing of chemically strengthened cover glass for portable electronic devices.
Some chemically strengthened glasses, such as some double-ion-exchanged glasses, are characterized by refractive index profiles containing a steep region of rapidly decreasing refractive index near the surface. Such index profiles are problematic for stress measurements using the established prism-coupling technique of industry-standard FSM-6000, due to significant broadening of the coupling resonances caused by excessive coupling between the light propagating in the prism and light propagating in the waveguide region of the glass substrate formed as a result of ion exchange.
Various techniques have been invented recently to mitigate this problem. In one method a low-index layer is used to control the coupling strength. This technique allows obtaining spectra of sharp high-contrast lines, but has the inconvenience of stricter requirements for cleanliness of the contacting surface due to need for proximity (typically <1 μm) between the glass and the measurement prism. Other methods include use of higher-index oil (>1.7) to increase the index contrast and narrow the coupling resonances, and inserting a diffuser in close proximity to the prism to further flatten the angular distribution of illumination, to counteract the high sensitivity of position detection of the broadened resonances to intensity variations in the background illumination. These latter methods have helped extend the range of the convenient high-index-oil measurement (no need for sub-micron thickness of the oil layer, no need for prevention of particles from the prism-sample interface), but the precision of such measurement is often poorer compared to measurements of traditional Gorilla® glass with moderate slope
                                    λ          n                ⁢                              ⅆ            n                                ⅆ            s                                      <    0.00016    ,where λ is the wavelength, n is the refractive index, and z is the depth coordinate. For example, substantially steeper profiles have showed standard deviations of the surface compressive stress (CS) often substantially greater than 20, even 50 MPa, which is problematic for obtaining high-yield quality control. In comparison, standard Gorilla® glass is quite often measured with CS standard deviation below 5 MPa. Hence, further methods of improving the measurement precision for profiles containing a shallow region of steeply decreasing refractive index with depth are of great value for enabling high-yields and allowing operation closest to the desirable design conditions for the chemically strengthened glass.